As one type gear pump, there is an internal gear rotary pump, such as a trochoid pump. One such internal gear pump has been disclosed in Japanese Patent Provisional Publication No. 11-117876 (hereinafter is referred to as “JP11-117876”). In the internal gear pump as disclosed in JP11-117876, a discharge pressure is introduced and applied to an outer peripheral portion (of a discharge port side) of an outer rotor via a high-pressure side communication line communicating a discharge port, whereas a suction pressure (an inlet pressure) is introduced and applied to an outer peripheral portion (of a suction port side) of the outer rotor via a low-pressure side communication line communicating a suction port. Owing to the pressure difference, the outer rotor is pushed to an inner rotor, thus reducing a clearance between the outer-rotor inner-toothed portion and the inner-rotor outer-toothed portion, in meshed-engagement with each other.
The inner rotor and the outer rotor are operably accommodated in a casing (a cam ring). During operation of the internal gear pump, that is, when the inner rotor is driven by a drive shaft, the outer rotor also rotates in the same rotational direction as the inner rotor by mesh between the outer-rotor inner-toothed portion and the inner-rotor outer-toothed portion.